1. Field of the Invention
The present invention relates to improvements in metal halide lamps and, more particularly, to improvements in metal halide lamps for use in optical instruments.
2. Description of the Prior Art
Metal halide lamps are used in optical instruments such as liquid crystal projectors and OHPs as incorporated in their reflectors. In recent years, to increase the screen brightness, metal halide lamps of the type having a pair of electrodes with a reduced spacing therebetween have been developed and widely used so that light from the lamp can be effectively utilized by a reflector. However, such a metal halide lamp causes an undesirable decrease in its own emission efficiency (lm/W) due to the reduced spacing between the electrodes. ("lm" means lumen.)
Attempts have been made to increase the emission efficiency of such metal halide lamp by encapsulating an excessive amount of a metal halide in the lamp. However, although the metal halide excessively encapsulated in the lamp enhances the emission efficiency of the lamp as intended, the screen brightness tends to decrease contrary to the expectation.
One conceivable reason for this problems is that the excessive metal halide encapsulated in the lamp does not sufficiently vaporize during the operation of the lamp and adheres to an inner wall surface of the lamp. This causes shadow and hence a decrease in the screen brightness.
Further, the metal halide excessively encapsulated in the lamp causes devitrification of the light-emitting tube formed of quartz glass and corrosion of the electrodes, which leads to the lamp with a shorter life time. Furthermore, convection occurring within the tube during the operation of the lamp is disturbed due to the excessive encapsulation of the metal halide, resulting in flicker.
It is, therefore, an object of the present invention to provide a metal halide lamp which is capable of increasing a screen brightness with less flicker and enjoys a longer life time by restricting the amount of a metal halide to be encapsulated in the lamp to a specific range.